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The abdominal muscles are strong bands of muscles that line the walls of the abdomen. They are located between the ribs and the pelvis, at the front of the body. There are five main abdominal muscles, which support the trunk, allow movement, and hold organs in place by regulating internal abdominal pressure. The deepest of these muscles is the transversus abdominis, which is responsible for abdominal compression. The internal obliques can also be used to compress the abdomen for exhalation.
| Characteristics | Values |
|---|---|
| Number of abdominal muscles | 5 main muscles |
| Location | Between the ribs and the pelvis |
| Function | Support the trunk, allow movement, hold organs in place, and regulate internal abdominal pressure |
| Muscle for compression of abdominal contents | Transversus abdominis |
| Muscle that assists in abdominal compression | Rectus abdominis |
| Muscle that can be used to compress the abdomen for exhalation | Internal obliques |
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Transversus abdominis
The transversus abdominis is a broad, paired muscular sheet found on the lateral sides of the abdominal wall. It is the deepest of the flat muscles, with transversely running fibres. It is also known as the transverse abdominis, transversalis muscle, and transverse abdominal muscle (TVA).
The transversus abdominis is layered below the internal oblique muscle. It arises as fleshy fibres from the lateral third of the inguinal ligament, the anterior three-fourths of the inner lip of the iliac crest, the inner surfaces of the cartilages of the lower six ribs, and the thoracolumbar fascia. It ends anteriorly in a broad aponeurosis, with its lower fibres curving medially and downward.
The transversus abdominis is important for maintaining normal abdominal tension and increasing intra-abdominal pressure. It serves to compress and retain the contents of the abdomen, providing thoracic and pelvic stability. It also assists in exhalation by contracting to force air out of the thorax.
The transversus abdominis is vital for back and core health. It helps to pull in what would otherwise be a protruding abdomen, earning it the nickname the "corset muscle". Training this muscle can help achieve a flat belly by bracing the human core during heavy lifts.
Weak transversus abdominis muscles can result in a bulge below the navel, an inability to hold in the stomach after a large meal or when gassy, and low back fatigue after prolonged standing or walking. The drawing-in technique is an effective way to activate and strengthen this muscle.
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Rectus abdominis
The rectus abdominis is a long, flat muscle that extends along the whole length of the front of the abdomen. It is separated from its fellow on the opposite side by the linea alba, a fibrous line that is visible as a vertical groove extending from the xiphoid process. The rectus abdominis is the top layer of the abdominal muscles, often referred to as the "six-pack". It is composed of two flat and parallel muscles, separated by the linea alba, a connective tissue.
The rectus abdominis is an important postural muscle that flexes the lumbar spine, as seen in crunches or leg-hip raises. It also assists with breathing and plays a role in respiration during forceful exhalation, such as after exercise. This muscle is paired and runs vertically on either side of the linea alba, on the anterior surface of the abdominal wall. It is attached by two tendons: the larger one connects to the pubic crest and pectineal line, while the smaller, medial tendon attaches to the pubic symphysis. The fibres of the rectus abdominis extend vertically superiorly and insert into the xiphoid process of the sternum and the costal cartilages of the 5th to 7th ribs.
The muscle is interrupted by tendinous intersections, which further subdivide it into smaller muscle bellies. These fibrous bands create a grid-iron "six-pack" shape in individuals with low body fat. The rectus abdominis is enclosed within the rectus sheath, a multilayered, fibrous compartment that also contains the pyramidalis muscle. The rectus sheath is formed by the merging of the aponeurosis of the transversus abdominis, external and internal oblique abdominal muscles.
The rectus abdominis is innervated by the thoraco-abdominal nerves, which are continuations of the T7-T11 intercostal nerves. These nerves pierce the anterior layer of the rectus sheath and provide sensory input from the 7-12 thoracic nerves. The blood supply to the rectus abdominis is predominantly provided by the inferior and superior epigastric arteries, along with contributions from other vessels.
The rectus abdominis can be isolated during exercise by bending the knees to minimise the engagement of hip flexors. Raising the trunk should be performed in a straight manner. Weak abdominal muscles can lead to spinal problems, so it is important to assess their strength rather than flexibility.
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Internal obliques
The internal abdominal oblique muscle is a broad, thin muscular sheet found on the lateral side of the abdomen. It forms one of the layers of the lateral abdominal wall, along with the external oblique on the outer side and the transverse abdominis on the inner side. The internal abdominal oblique muscle is important for maintaining abdominal pressure and movements of the trunk, along with the other abdominal muscles.
The internal obliques are sandwiched in the middle between the external obliques and the transversus abdominis. They begin from the side of the hip crest and the inguinal ligament and connect with the linea semilunaris. Their fibres are obliquely oriented, perpendicular to those of the external oblique. The internal obliques are also referred to as "same-side rotators" as they cause ipsilateral rotation and side-bending when contracted.
The internal obliques act as an accessory muscle of respiration, helping to reduce the volume of the chest cavity during exhalation. When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs, which then fill with air. Conversely, when the internal obliques contract, they compress the organs of the abdomen, pushing them up into the diaphragm, which intrudes back into the chest cavity, reducing the volume of the air-filled lungs and producing an exhalation.
The internal obliques also work with the external oblique muscle on the opposite side to achieve torsional movement of the trunk. For example, the right internal oblique and the left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip.
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External obliques
The external oblique muscle is one of the outermost abdominal muscles, extending from the lower half of the ribs around and down to the pelvis. It is the largest and most superficial of the lateral abdominal muscles. External abdominal oblique muscles are paired muscles located on the lateral sides of the abdominal wall. They cover the sides of the abdominal area, sitting on the top surface of the abdomen right below the subcutaneous fat and skin.
The external oblique muscle functions to pull the chest downwards and compress the abdominal cavity, which increases intra-abdominal pressure. This compression is essential for "voiding" various things from the body, such as defecation, urination, vomiting, and childbirth. The muscle also performs ipsilateral (same-side) side-bending and contralateral (opposite-side) rotation: the right external oblique would side-bend to the right and rotate to the left, and vice versa.
Bilateral contraction of the external oblique muscle, along with the rectus abdominis and internal oblique, flexes the trunk by drawing the pubis towards the xiphoid, similar to crunches or sit-ups. This bilateral contraction increases intra-abdominal pressure, which is useful in processes such as breathing, singing, and defecation.
The external abdominal oblique is innervated by the anterior rami of the thoracic spinal nerves T7-T12. The intercostal nerves T7-T11 supply the superior-most part of the muscle, while the subcostal nerve T12 innervates the lower part. The muscle receives sensory innervation from the lumbar plexus, specifically the iliohypogastric nerve (L1). The upper two-thirds of the external oblique receive blood supply from the branches of the lower posterior intercostal and subcostal arteries, while the lower third is supplied by the deep circumflex iliac artery.
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Pyramidalis
The pyramidalis is a small, triangular muscle that is part of the anterior abdominal wall. It is found on each side of the linea alba, a fibrous line that runs down the centre of the abdomen. The linea alba is a centre of the abdominal aponeurosis, a sheet of tendon that the muscles of the front of the abdomen are attached to one another and the skeleton. The pyramidalis muscle is contained within the rectus sheath, a multilayered fascial compartment composed of the aponeuroses of the external abdominal oblique, internal abdominal oblique, and transversus abdominis muscles.
The pyramidalis muscle is not always present, being absent in about 20% of cases, and can vary in size. It is a vestigial muscle, meaning it is tendinous in larger part or reduced in size compared to homologous muscles in other species. The muscle is innervated by the subcostal nerve, which is the anterior ramus of spinal nerve T12, and receives its blood supply from branches of the inferior epigastric artery.
The pyramidalis muscle originates from the pubic symphysis and pubic crest, with ligamentous fibres arising from the symphysis and tendinous fibres from the bony attachment at the crest. The muscle belly narrows as it courses superiorly and inserts into the linea alba, midway between the umbilicus and pubis. The linea alba is tensed by the contraction of the pyramidalis muscle, which also contracts with the other abdominal muscles to increase positive abdominal pressure. This dual action serves as a defence mechanism to protect the abdominal organs and support physiological processes such as forced respiration, singing, micturition, and defecation.
The pyramidalis muscle is also used as a surgical landmark, such as in a classical caesarean section, to determine the midline and location of the linea alba. Additionally, it has been used as a source of striated muscle stem cells for the treatment of post-prostatectomy stress urinary incontinence.
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Frequently asked questions
The five main abdominal muscles are the rectus abdominis, pyramidalis, external obliques, internal obliques, and transversus abdominis.
The rectus abdominis is a pair of muscles that run down either side of the middle of the abdomen. They are divided into two segments by the linea alba. The rectus abdominis holds internal organs in place and keeps the body stable during movement.
The transversus abdominis is the deepest abdominal muscle. Its contraction causes compression of the abdominal contents.
The pyramidalis is a small, triangular vertical muscle located at the base of the pubic bone. It helps to maintain internal pressure in the abdomen.
The internal and external obliques are abdominal muscles that work together to compress the abdomen for exhalation. They also play a role in flexing or twisting the spine.
